# Does the prefix Ortho have some significance? [duplicate]

Are orthoboric acid and boric acid one and the same thing or we can not write H3BO3 as B(OH)3?

The reference given by Andselisk does not bring an answer to the question about the origin of the prefix "ortho" in inorganic nomenclature.

The prefix "ortho" is added before the name of an acid $$H_aX_bO_c$$ if first $$b = 1$$ and if it corresponds to the highest possible number of $$OH$$ groups attached to the central X atom. For example $$H_4SiO_4$$ is the orthosilicic acid, $$H_3BO_3$$ is the orthoboric acid. $$H_4CO_4$$ is the orthocarbonic acid (which only exists in esters). $$H_3PO_4$$ is the orthophosphoric acid (where the prefix is often skipped). $$H_3AsO_4$$ is the orthoarsenic acid. The use of the prefix does not depend on the oxidation number of $$X$$. For example, if $$P$$ and $$As$$ are taken with a lower oxidation number, one obtains $$H_3PO_3$$ as the orthophosphorous acid, and $$H_3AsO_3$$ as the orthoarsenous acid.

The prefix "meta" is added to the name of an acid, if its formula is formed by removing one molecule $$H_2O$$ from the formula of the ortho-acid. $$H_3BO_3$$ - $$H_2O$$ = $$HBO_2$$ and this is the metaboric acid. $$H_3PO_4$$ - $$H_2O$$ = $$HPO_3$$ which is the metaphosphoric acid. Repeating this process, one gets :$$H_2CO_3$$ is the metacarbonic acid (often written without the prefix meta). $$H_2SiO_3$$ is the metasilicic acid. As with ortho-acids, the use of "meta-" does not depend of the oxidation number. $$HPO_2$$ is the metaphosphorous acid. $$HAsO_2$$ is the metaarsenous acid.

It should also be mentioned that removing one $$H_2O$$ out of two molecules of an ortho- acid gives a pyro- acid. Example : $$2 H_3PO_4 - H_2O = H_4P_2O_7$$ and this is the pyrophosphoric acid. $$H_4As_2O_7$$ is the pyroarsenic acid. This prefix pyro- is even used for acids existing only under the ortho- definition. For example, a pyro derivate from the sulfuric and sulfurous acids $$H_2SO_4$$ and $$H_2SO_3$$ exist. They are the pyrosulfuric acid $$H_2S_2O_7$$. and the pyrosulfurous acid $$H_2S_2O_5$$

It should also be mentioned that the prefixes ortho-, meta-, and pyro- are never used with acids having already a prefix like hypo- and per-.

All these rules have been edited in the middle of the 19th century.

• I admit the link I provided wasn't the most relevant one, thank you for pointing this out. I corrected my comment and included the link that IMO fits better. Either way, the question appears to a be a duplicate. – andselisk Jan 8 at 13:54